1) Field of the Invention
This invention pertains to the field of characterization and measurement of human and animal visual performance, and more particularly, to a method of performing objective measurement of human and animal visual performance.
2) Description of the Related Art
Traditional measures of human and animal visual performance have been limited by the need for subjective feedback from patients, or to examination of a limited number of parameters. The Snellen chart is often used to measure the visual acuity of the subject. The visual acuity is defined in terms of the ability to distinguish letters or shapes in a standard chart, the size of the letters that can be recognized being a measure of the optical system performance of the eye.
Unfortunately, the most common method for measuring visual performance, subjective refraction, cannot be applied to animal subjects used for research, for very young children, or those with learning or communication difficulties. New instruments have been developed in recent years to make objective measurements of the eye. These instruments include: autorefractors for measuring the basic refractive error; corneal topography instruments that can measure the corneal surface; and a variety of aberrometers that have recently been shown to be able to measure higher order aberrations. With each new advance, more information can be measured about the eye.
Auto-Refractor.
The auto-refractor is an instrument that uses one of a variety of methods to analyze the cornea and lens of an eye to determine the basic refractive error. This error is presented in terms of the spherical and cylindrical power of a lens that is needed for its correction. In the event that higher order irregularities are present, these are included only in the effect that they have on the overall visual performance.
Corneal Topography.
A corneal topography instrument uses one of a number of techniques to measure the shape of the cornea. Since this shape is, in many cases, the source of the aberrations, it provides a useful and more complete measure of the optical performance of the eye. The information can be used to predict the visual acuity and optical performance with greater accuracy than using the sphero-cylinder terms alone. However, assumptions about the interior structure of the eye must be made in inferring the visual performance from corneal topography measurements alone.
Aberrometer (Also Known as Wavefront Analyzer).
The aberrometer is a system for measuring the aberrations of the eye by projecting light onto the retina and thus observing the effect of the entire ocular optical system. Using these methods both the basic refractive error (sphero-cylinder) and higher order aberrations may be determined. However, since this represents a measurement of the full optical path through the eye, a complete estimate of all the important optical effects can be obtained. It is not necessary to make assumptions about the internal or external structure of the eye since the aberrations can be measured directly. An objective aberrometer does not rely on subjective feedback, but measures the aberrations by projecting light into the eye and measuring it using objective means.
Given this new level of accurate information that can be determined about the eye, it remains an important task for the clinician to interpret the data in light of both the historical measures, and in terms that can be readily understood by patients. For example, Mrochen, Kaemmerer, Mierdel and Seiler used an objective aberrometer to measure the effect of laser refractive surgery on ocular aberrations. Mrochen, Kaemmerer, Mierdel and Seiler, 27 J. CATARACT REFRACT SURG. (March 2001). But while they were able to relate their results in terms of the optical aberrations of the eye, they were not able to make the connection to patient satisfaction or degradation in visual acuity in scotopic conditions. While the aberrometer is capable of providing a precise wavefront map of the aberrations of the optical system, unless the information can be interpreted by the clinician or researcher, it may have little value for diagnosis and treatment.
There are a number of methods currently used to measure performance of the ocular system. The most widely used and well established are psycho-physical methods, i.e., methods relying on subjective patient feedback. The oldest of the psycho-physical methods is the phoropter or trial lens method, which relies on trial and error to determine the required correction. There are psycho-physical methods for measuring visual acuity, ocular modulation transfer function, contrast sensitivity and other parameters of interest. While new advances in this field continue (see, e.g., PCT Publication No. WO 93/002614, European Patent Number 00600963/EP B1), they rely on subjective feedback and hence suffer from the limitations cited above.
In addition to these subjective methods, there are also objective methods for assessing the performance of the ocular system. Such objective methods include corneal topography, wavefront aberrometry, corneal interferometry, and auto-refraction. Many of these methods only measure the contribution of specific elements to the total refractive error. For example, much work has been directed to measuring the topography of the cornea and characterizing the corneal layer. However, the corneal shape only contributes about 30-40% of the total refractive error in most cases. In order to measure the bulk of the refractive error and to provide a complete mapping for diagnosis and correction, additional information and measurements are needed.
Another method for determining the refraction of the eye is auto-refraction, which uses a variety of techniques to automatically determine the required corrective prescription. These automated techniques include projecting one or more spots or patterns onto the retina, automatically adjusting optical elements in the auto-refractor until the desired response is achieved, and determining the required correction from this adjustment. However, auto-refractors are not considered especially reliable. Further, auto-refractors measure only lower order components of the aberrations, e.g., focus and astigmatic errors.
Recently, the eye has been considered an optical system, leading to the application of methods previously used for other optical systems to the measurement of the eye. These methods include interferometry and Shack-Hartmann wavefront sensing. These techniques are of particular interest because they measure the complete aberrations of the eye. These include the low order aberrations such as defocus and astigmatism, as well as higher order aberrations such as coma, spherical aberration or other more rapidly varying components. This additional information allows measurement of non-uniform, asymmetric errors that may be affecting vision. Further, this information may be linked with any of the various corrective techniques to provide improved vision. For example, U.S. Pat. No. 5,777,719 to Williams describes the application of Shack-Hartmann wavefront sensing and adaptive optics for correcting ocular aberrations to make a super-resolution retina-scope. U.S. Pat. No. 5,949,521 to Williams et al. describes using this information to make better contacts, intra-ocular lenses and other optical elements. Several types of objective aberrometers have been developed which can provide the wavefront information. These include the Shack-Hartmann (or Hartmann-Shack), Moirxc3xa9 deflectometry, Tscheming aberrometer, scanning resolved refractometer (ray-tracing), and a variety of other wavefront aberrometer technologies.
Wavefront aberrometry measures the full, end-to-end aberrations through the entire optics of the eye. In these measurements, a spot is projected onto the retina, and the resulting returned light is measured with an optical system, thus obtaining a full, integrated, line-of-sight measurement of the eye""s aberrations. U.S. patent application Ser. No. 09/692,483 describes a method for designing practical clinical instruments for measuring full, end-to-end aberrations through the entire optics of the eye.
Williams, and others, have been able to use a full adaptive optics system to present a corrected eye chart to a subject. However, this is still not useful for determining the objective visual acuity. While such a system can produce images with arbitrary effects included (up to the mechanical ability of the deformable mirror components), it introduces a significant additional cost and complexity.
Camp, McGuire, Cameron and Robb attempted to simulate the image quality and visual performance based on a model of the internal structure and measurements of the corneal surface. 109 AM. J. OPHTHALMOLOGY 4 (Apr. 15, 1990). Klonos, Pallikaris and Fitzke extended this work using an improved model of the internal structure. 12 J. REFRACT. SURG. 2 (February 1996). Barsky, Garcia, Klein, and Van De Pol reported a method for computing the visual acuity from the wavefront inferred from corneal topography measurements and a model of the interior structure of the eye in SPIE 3591, pp. 303-310 (1999). In these cases the internal structure of the eye was simulated based on a priori assumptions.
While ray-tracing and other techniques have attempted to predict visual acuity based on the refraction or corneal topography data, in all cases these relied on assumptions, rather than measurements, of the ocular optical system.
Accordingly, it would be advantageous to provide a method is needed to predict visual acuity and other ocular performance metrics from the aberrometry data. In particular, it would be advantageous to provide a method of evaluating of visual performance that relies on only measured values for the ocular aberrations. It would also be advantageous to provide a method for computing the ocular performance, predicting the appearance of images viewed by a subject, and determining the visual acuity in an objective fashion. Other and further objects and advantages will appear hereinafter.
The present invention comprises a method for computing the visual performance of a human or animal subject by using measurements of the subject""s aberrations obtained from an aberrometer that measures the full aberrations through the ocular optical system.
In one aspect of the invention, a method for computing the visual performance of a human or animal subject based on objective measurements of visual refraction including higher order aberrations, includes measuring wavefront aberrations of a subject ocular pupil, computing a point-spread-function from the measured pupil aberration, providing a test image, and convolving the test image with the point-spread-function. Beneficially, a simulated image, that is similar to a subject""s view, is produced from the convolution result of the test image with the point-spread-function. Also beneficially, the convolution of the test image with the point-spread-function includes performing an inverse Fourier transform of a modulus squared of the point-spread-function to compute an optical transfer function, computing a Fourier transform of the test image, and computing an inverse Fourier transform of a product of the optical transfer function and the test image Fourier transform.
Moreover, in another aspect of the invention, the subject""s visual acuity and visual acuity number can be determined objectively from these computations. In that case, the test image may be a Snellen acuity chart.
In another aspect of the invention, a method for computing an image, includes measuring wavefront aberrations of a subject ocular pupil, computing a point-spread-function from the measured pupil aberration, providing a test image, convolving the test image with the point-spread-function, and producing a simulated image from a convolution result of convolving the test image with the point-spread-function, wherein at least one specific term of the point-spread-function is adjusted prior to the convolving step to simulate an effect of a correcting means. Beneficially, adjusting the specific term comprises subtracting or eliminating the specific term, e.g., by setting a high order Zernike coefficient in the pupil aberration function to zero.
In yet another aspect of the invention, a method for computing an effect of corrective means on a subject""s visual acuity, includes measuring a subject pupil aberration function with an objective aberrometer, adjusting (e.g., adding or subtracting) terms to the measured subject pupil aberration function that correspond to specific corrective means, producing a modified pupil aberration function, computing a point-spread-function from the modified pupil aberration function, computing an optical transfer function (OTF) of the computed point-spread-function, multiplying a Fourier transform of a test image by the OTF to produce a Fourier result, and performing an inverse Fourier transform on the Fourier result. The corrective means may be spectacle lenses, contact lenses, laser refractive surgery, or Laser Thermal Keratotomy.
In still another aspect of the invention, a method of determining a best correction for a given subject, includes measuring a subject pupil aberration function with an objective aberrometer, subtracting terms from the pupil aberration function that correspond to specific corrective means, producing a modified pupil aberration function, computing a point-spread-function from the modified pupil aberration function, computing an optical transfer function (OTF) of the computed point-spread-function, multiplying a Fourier transform of a test image by the OTF to produce a Fourier result, performing an inverse Fourier transform on the Fourier result, and adjusting the terms that are subtracted to optimize a resultant image.